The US State Department is investigating reports that an American man has been detained in North Korea for "rash behaviour".

The 24-year-old had tried to enter the country on a tourist visa but tore it up and shouted he wanted asylum, according to state media.

The official Korean Central News Agency (KCNA) said he chose the North "as a shelter".

He was held two weeks ago, added the agency which identified him as Miller Matthew Todd.

It said he was detained for "gross violation" of North Korea's legal order and was being investigated.

The announcement came as US President Barack Obama was visiting South Korea, one of Washington's closest allies and still technically at war with Pyongyang.

Mr Obama warned the North against conducting another nuclear test.

"Threats will get North Korea nothing, other than greater isolation," the US leader said, speaking at a joint news conference with South Korean President Park Geun-hye.

North Korea's young leader Kim Jong-Un

Mr Obama said the US and South Korea stand "shoulder to shoulder" in refusing to accept North Korea's nuclear programme.

Pyongyang threatened last month to conduct a fourth nuclear test, possibly while Mr Obama is in the region.

The KCNA said: "A relevant organ of the DPRK put in custody American Miller Matthew Todd, 24, on April 10 for his rash behaviour in the course of going through formalities for entry into the DPRK to tour it."

State Department spokeswoman Jen Psaki says the US is in touch with the Swedish Embassy.

Sweden handles consular cases for the US because Washington does not have diplomatic relations with Pyongyang.

While KCNA identified the man as Miller Matthew Todd, it appeared it had used the Korean convention of putting the last name first, followed by the first and middle names.